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Full text of "Handbook Of Chemical Engineering - I"

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Volume, per cent of full load..............    0   20     40   50   60   70   80 100 12
Pressure, per cent of full load............. 92   94 97.5 100 103 105 105 100   9J
Power, per cent of full load............... 50   53     58   62   66   73   81 100 120
Rotary blowers are built for air pressures varying from 6 oz. to 10 Ib. or even 12 Ib. per square inch. The best efficiencies of this type of blower, however are usually secured below 5 Ib. pressure, but the simplicity of the machine gives it an advantage over compressors of the piston type and frequently warrants its installation for the higher pressures indicated when designed for this purpose. As the machine operates by displacement, it is usually preferred for cupola practice, and copper and lead blast furnaces, because its positive action will not permit 2 reduction in air supply if the cupola tends to clog. For other uses of air at pressures below 8 oz. the fan is ordinarily more economical.
Blowers of this type may be arranged to give either constant volume or constam pressure, and to handle either liquids or gases. They consist of a casing containing one or more revolving impellers of various forms of design. Figure 36 represents a cross-section of the Sturtevant high-pressure blower, which is built in capacities ranging from 5 to 15,000 cu. ft. per minute at 8 oz. pressure (speeds, from 375 to 800 r.p.m. for the smallest size down to 160 to 220 r.p.m. for the largest; weights with sub-base range from 200 to 39,000 Ib.). The smaller machines have a vertical arrangement of shafts, while the larger types usually have their shafts in the same horizontal plane. Two impeller blades are always in action, and leakage by one is caught by the other. The proper size of blower for a cupola may be calculated on the basis of 80,000 cu. ft. of air per ton of iron melted. Sturtevant machines are also used or handling gases. The capacities for the various sizes of gas exhausters range from 7,500 to 900,000 cu. ft. per hour at 8 OK. pressure, making no allowance for shrinkage, which will vary from 10 to 20 per cent, depending on the gas and its pressure. The inlet and outlet diameters run from 3 in. in the smallest size to 30 in. in the largest; weights, from 400 to 33,000 Ib. Figure 37 illustrates a cross-section of a Roots blower. The two impellers are symmetrical and are driven in opposite directions by gears outside the casing. The impellers do not touch each other nor the casing, but the clearance is reduced to a minimum in order to reduce slip or leakage. The amount of this slip or leakage may be determined by operating the machine with a closed discharge, at a speed sufficient to maintain the required discharge pressure. The amount is usually largest in machines of smallest capacity, i.e., a machine displacing 0.75 cu. ft. per revolution at a pressure of 1 Ib. will have a slip of from 60 to 70 revolutions, while a machine having a capacity of 300 cu. ft. per revolution will have a slip of from 3 to 5 revolutions. For intermediate capacities the slip will vary proportionally and increase with higher pressures as the sqiia-rxj root of the discharge pressure, i.e., at 4 Ib. pressure the slip will be approximately twice that at 1 Ib.
In most blower work the so-called hydraulic formula for horsepower will be found satisfactory: horsepower « Q(p2 - pi)/33,000, where Q is the cubic feet of
FIG. 36.—Sturtevant rotary blower.
FIG. 37.—Roots blower.